1. Field of the Invention
The present invention relates to an electronic throttle control for an internal combustion engine, provided with a function for detecting failure of a throttle sensor used to detect the opening of a throttle valve (throttle opening).
2. Related Art
In an electronic throttle system mounted on a vehicle, how much the accelerator pedal is depressed (accelerator depression) is detected by an accelerator sensor and a target throttle opening is set according to the detected accelerator depression and a motor for driving the throttle valve is feedback-controlled (feedback control) so that the actual throttle opening detected by the throttle sensor matches with the target throttle opening.
In such an electronic throttle system, when a throttle sensor failure is detected, an electromagnetic clutch connecting the motor to the throttle valve is turned off to stop the throttle control for the purpose of fail-safe as disclosed in JP-A 4-350332. The vehicle begins limp-home running by interlocking the throttle opening mechanically with the accelerator pedal operation and displays a warning message indicating the failure.
The throttle sensor, even when it is normal, outputs signals on the same level as that of failure time signals due to signal noises, instantaneous break, etc. Since such a state is instantaneous, however, the normal state is restored immediately. In the above arrangement, if the throttle system detects even an instantaneous output change caused by noises, instantaneous break, etc. as a false failure the vehicle begins limp-home running immediately and displays the message indicating the failure. At this time, the output from the throttle sensor is already restored to its normal state. In such a case, the vehicle begins limp-home running and displays the warning even when the throttle sensor is normal, so that a driver would feel that the system is not reliable so much.
To prevent such a problem, a determination delay time is set longer than the signal width assumed for the noise, instantaneous break, etc. for determination of throttle sensor failure. When the failure output of the throttle sensor continues over the determination delay time, it is determined finally that a failure has occurred. Thus, false sensor failure detection caused by noise, instantaneous break, etc. can be prevented effectively.
The measure for preventing such detection failure, however, will continue the feedback control for the motor in response to the abnormal output from the throttle sensor until the preset determination delay time is over, even when the throttle sensor is in an abnormal state. As a result, the fail-safe operation is delayed, thus degrading the fail-safe operation of the system.